Cerulean (/səˈruːliən/), also spelled caerulean, is a color term that may be applied to certain colors with the hue ranging roughly between blue and cyan, overlapping with both. It also largely overlaps with azure and sky blue, although cerulean is dimmer.

The first recorded use of cerulean as a color name in English was in 1590.[2] The word is derived from the Latin word caeruleus, "dark blue, blue or blue-green", which in turn probably derives from caelulum, diminutive of caelum, "heaven, sky".[3]

A sample swatch of cerulean blue hue oil paint. 'Hue' in this case means that other pigments have been used to mimic the hue of oil paint containing the original pigment.

In classical times, cerulean was used to describe blue pigments, particularly mixtures of copper and cobaltousoxides. These early attempts to create sky blue colors were often less than satisfactory due to greenish hues and lack of permanence. When the pigment cerulean blue (shown in the color box to the left) was invented, it largely superseded all these prior pigments. See also Tekhelet.

The first recorded use of cerulean blue as a color name in English was in 1859.[5]

Pigments through the ages shows a "Painted swatch of cerulean blue" that is representative of the actual cobalt stannate pigment. This color swatch matches the color shown in the color box at right.[6]

It is particularly valuable for artistic painting of skies because of the purity of the blue (specifically the lack of greenish hues), its permanence (no other blue pigments retained color as well), and its opaqueness.[10]

Today, cobalt chromate is sometimes marketed under the cerulean blue name but is darker and greener (Rex Art color index PB 36) than the cobalt stannate version (color index PB 35). The chromate makes excellent turquoise colors and is identified by Rex Art and some other manufacturers as "cobalt turquoise".[11][12]

^The color displayed in the color box above matches the color called cerulean blue in the 1930 book by Maerz and Paul A Dictionary of Color New York:1930 McGraw-Hill; the color cerulean blue is displayed on page 89, Plate 33, Color Sample L9.